555 signal generator + LM358 op amp + recycled 8ohm speaker


This circuit is from page 19 of Forrest M. Mimm’s Engineer’s Mini-Notebook : 555 TImer IC Circuits.

I am using only a 0.1uF capacitor and don’t have any of the buttons shown in this circuit.

This circuit is from page 259 of Learning the Art of Electronics: A Hands on Lab course by Thomas C. Hayes.

I am taking the output from the 555 circuit and feeding it to the + input of an LM358 operational amplifier. Unlike the LM386 which is optimized for audio amplifying, the LM358 needs a push pull to amplify the signal.

I’m using an 2N2222 and a 3906 as transistors and a recycled speaker as output. I added a 270 ohm resistor between the push-pull an the output of the LM358 as well as a 100K resistor in the negative feedback loop, as is done elsewhere in Learning the Art of Electronics.

The yellow is the signal produced by the 555 and the green is the amplified (and inverted – because we’re using an inverting op amp configuration) signal. But I think there is a mistake here because I don’t see any amplification here, just inversion!

I tried again with a different opamp and circuit configuration. This is the LM386 which is optimized for audio amplification of digital signals.

I am using a circuit from p.920 of The Art of Electronics but it is essentially the same as the base circuit suggested in the LM386 datasheet:

I’m running out of ways to take interesting photos of breadboards but here’s another go:

And here is a look on the scope, the green is the input signal (2.5V 1KHz demo signal from oscilloscope) and the yellow is the amplified signal. This circuit is supposed to have a gain of 20, gain being the ratio of Vout/Vin. This scope image would appear show a gain of under 2 though… In any case pins 1 and 8 allow to increase the gain to higher values and increasing the supply voltage of the LM386 up to 12V would allow for even more amplification. The only trick at that point is checking to see what the range of voltage is for the input signal and about what voltage it is centered (say ground, for instance).